It is known in the art of fiber lasers that a fiber laser comprises a length of optical fiber (or laser cavity) which is doped with an optically active rare-earth ion (or gain medium), e.g., Neodymium or Erbium, and has optical reflectors spaced apart by a predetermined distance along the fiber with the gain medium there between. The fiber is optically pumped by pump light having a predetermined pump wavelength which excites the gain medium such that the population of excited atoms is greater than the population of unexcited (or less excited) atoms in the lasing transition (known as population inversion). As the energy of the atoms in the gain material transition back to their original unexcited state (or a lower energy level), photons are emitted at a predetermined lasing wavelength. Such emitted photons cause (or stimulate) other excited atoms in the gain medium to emit similar photons, thereby creating the well known lasing effect. The optical reflectors are designed to reflect a predetermined amount of light at the lasing wavelength and the length of the cavity and the amount of cavity gain is set so as to cause light at the lasing wavelength to continuously oscillate within the cavity to allow lasing to be sustained. Also, at least one of the reflectors does not reflect light at the pump wavelength, thereby allowing the pump light to enter the cavity through one of the end reflectors.
It is also known that such reflectors may be Bragg gratings which are impressed directly into the optical fiber, as discussed in U.S. Pat. Nos. 4,807,950 and 4,725,110 entitled "Method for Impressing Gratings within Fiber Optics", both to Glenn et al.
Such a laser can be designed and fabricated so as to achieve single longitudinal mode lasing performance with narrow linewidth and continuous tunability over a predetermined wavelength range, as is discussed in U.S. Pat. Nos. 5,305,335, entitled "Single Longitudinal Mode Pumped Optical Waveguide Laser Arrangement", to Ball et al, and U.S. Pat. No. 5,317,576, entitled "Continuously Tunable Single-Mode Rare-Earth Doped Pumped Laser Arrangement", to Ball et al.
Such fiber laser sources offer the possibility of improved performance characteristics such as higher power and narrower linewidth when compared to semiconductor laser sources and diode pumped solid state laser sources commonly used in fiber optic systems.
However, it is also known that the intensity of the output light from a fiber laser may exhibit variations with time (or noise). This noise is called relative intensity noise (RIN) and is typically measured in dB/Hz with respect to the continuous wave (cw) lasing level and has a magnitude profile which varies with frequency, as is known. For example, for a prior art Erbium-doped fiber laser, the RIN may be -110 dB at low frequencies (e.g., less than 100 KHz) and -140 dB at high frequencies (e.g., greater than 100 MHz).
While this level of noise may be acceptable for digital systems, it is not acceptable for analog applications such as cable television, which require a noise level of about -160 dB/Hz at high frequencies.
Also, there exists a localized resonant peak (or noise spike) in the RIN profile at low frequencies (e.g., approximately 250 KHz, depending on the laser power) which is related to relaxation oscillations in the laser cavity. This peak has a magnitude of about -80 dB/Hz in some prior art systems. It is desirable for digital and analog transmission systems to reduce the RIN noise spike as much as possible, e.g., to less than -120 dB/Hz.
One way to decrease RIN is to increase the laser output power. One technique known in the art for increasing laser power is to increase the cavity doping concentration. However, such increased concentration often leads to clustering effects in the Er which reduces laser efficiency and causes self-spiking.
Another way to increase power is to use a more efficient gain medium, e.g., a co-doped Erbium-Ytterbium fiber as described in the article: Kringlebotn et al, "Efficient Diode-Pumped Single-Frequency Erbium:Ytterbium Fiber Laser", IEEE Photonics Techn. Lett., Vol. 5, No. 10, pp 1162-1164 (October 1993); and J. Kringlebotn et al, "Highly-efficient, Low-noise Grating-feedback Er3+:Yb3+Codoped Fibre Laser", Electr. Lettr., Vol. 30, No. 12, pp 972-973 (June 1994). Even though such increased power reduces the overall RIN at all frequencies, such technique does not eliminate the low frequency RIN peak. Also, as laser power increases, the frequency where the RIN noise spike occurs increases as well.
Thus, it would be desirable to provide a fiber laser which has reduced RIN profile at both low and high frequencies.